Noise reduction amplifier circuit for sound recording systems



April 15, 1947- A. BADMAIEFF 2,419,001

v NOISE REDUCTION AMPLIFIER CIRCUIT FOR SOUND RECORDING SYSTEMS Filed March 1o, 1945 6 1@ v w Il@ ls J' ATTORNV Patented Apr. 15, 1947 Alexis Badmaie, L`s'Angeles,Calf-, assignerA toy Radio Corporationmof, America-a r corporation of Delawarev Application Marchv`, 1945SerliallN: 5822137 (Cl. 17a-100.3).-

11- Claims;

'This invention relates tok current controisys tems,1and particularly to rectier-lter combina`V tionsfor translating alternating currents intol di:

rect currents having predetermined voltage'v'ari-r ations with respect to the alternating 'currentvar--Y iations':

In the art. of sound recording wherein alternatingpcurrent" signals are recorded on' film, noise reductionzis generally applied in .one form or another; In thexapplication ofY noise reduction,`it

iseusually'necessary to rectify'the signal currents; and vpass'tl'ierectied; componentsthrough a l`: ter before impression upon the noise reduction'` element of the; recording system, the noisevre-y duction; elementsvarying withl the: type of recording system. For instance, the noise `reducition' current may be usedfor varying the'. position'of, alight interceptingv shutter; -a galvanometer reiiecting mirror; o-r light valve ribbons; in accordancevvith the average amplitudeyaria'- tions'of'tliesignal. The lter circuits are usedto veliminate the'ripple components and'toimfpart theproper movements to these elements dur-l-l inginc-reasesand decreases in the alternatingfcurrent signal." varies the average .value of the light beam impressed on. the photocell during reproduction',v

SinceVv the noise reduction signal tliese light variations should notv be so rapid that theyf themselves introduce noise" into the reprow ducedrsound,` but they inust be sufficiently' rapid to avoid; clipping a substantial number' ofi the," peaks-of theinitial portion of the signal during recording.` To' avoid introducing noise" the form of thumps, vthe movementof the: noise reduction .i elements should provide a Waveform; in which there :is no yfrequencyabovethe cut ofl frequency of the reproducing system.

There have-been: many circuits suggestedfor' controlling noise reduction elementsinisoundre'e cording, andreference is hereby made to Kreuzer.

Patents No; 2 224,914 ci December 17,' 1940; and" No. 2,260,717 fof October'28, 1941, Whchdisclosef and claim ,certainwtypes ofitnoise' reductioniam-v pliiercircuitsto obtain certain results ,duringnf-- creasesin afsigna'l.'v Kellogg patentsfNo. 2,227,906* OfJanuaryl, 1941, and No.,2,357,661 of September.

5, ,1944, alsoshow otherytypes of combination rec:

tier-fllter'circuits'for controlling noise reduction. elements.` The 1 present invention; however,V has?l af closing;Y characteristic4 similar; to the noise -reduction; circuits. shoWnin Haynes .Patents No. 2,357,652. of vSeptember 5,1944', and No.y 2,359,989? of TOctob'enl, 1944, althoughr improved-operating; resultsare obtainablelwith the present circuitsg:`

aszwillberpointed out-hereinafter:

S'omeeof fthel chief adwantages` of the invention@L are the elimination of the effect of resonancein'l the;v mechanical noise reduction element, more rapid opening-action `to reduce' peak' clipping', ,and the'fiprovi'siorrof a rectifier-directr current ampli-` ercharacteristic. to producean' average) light.'

variationy which doesy not contain frequenciesV above ther cut'. 'off 1 'frequency' of the reproducing, system; A feature ofi' theinvention is the are' rangement of' the,frectier'1ter. combination to,Y avoidI-theeiect of leakage in. the windings ofthe rectierstransformer, wl'iile-providing a desirable? linear closing-'characteristic:when the alternating? current: signal ceases or decreases-.1

The: principal i object of' the: invention,- there` foreiis,toi-.facilitate themecordinggr of: sound with' noise-reduction'.

Anothen'obect ofthe invention isto'provide'an: improvedfnoi'se reduction system which.. neutraliizesaresonance fof ther mechanical load;

Aiurtherfobiect ofthe invention :is toiprovide-7 an?. improved rectier-filten combination'. which` produces a" voltage variationxhaving; frequencies',- belowitlie'cut oif frequency ofthereproducing'.V system;

Azfurtherfobject of the inventionis'to provide"y arrectier" circuiti having a .predetermined creia-f tionshipbctween: itsf'input and loutput voltages:

-A stilltfurther'objectY of tli'e'invention is toproz-fV videira rectifler-lter v'combination which provides 2:: rapid. opening L characteristic and a; linear clos ing;v characteristic" and'V in:- which the.: rectifierv isl usedasa limiter.-

Although .the Anovel features` which arefbelievedz;

tobefcliaracterlstic offthi'sinvention willibe point edoutzwith 'particularity linthe f appended claims,V themann'eri offV its organization.` and the" modef of 1 itsifoperation'willi'.be better'understood by refer ringit'o the following description read inconjunc tion with the accompanying drawings'forming?a.:A

pahereofjmwhioh:

lsisiazschematicdrawing,of afrectieriilter" circuitfinswhicha the principles of?theninventi'on*y Figa; 2: `isfaischermati-c-circuit `:off anew noise lreduction amplie-r embodying 'thefprinciples'of v"theI Fig;y 3 issa graphnshowingftheshutterA opening.- characteristic; and if 's Fig: 4i.isi'agraplishowingzthe. shutter? closing; character-istimf.

nie'ntsiofiv a ,-cornrnerciarv types off' noise: reduction-f circuit'iare ashowna'ha'lfiwaye rectifier: 5-rece1vesf alter-noting.currentfromfa; transformer secondary pressed on a coil'v I5 which mayv be the activating" coil for noise reduction shutters or the noise re-v duction bias coil of a galvanometer. It has been found that due to the resonance of a shutter load;

the movement of the shutter is irregular, that is, u, Y contains a ripple above the cut off frequency ofy` the reproducing system and is thus audible when the sound record is reproduced.l In the case o f the shutter, this resonance frequency is approximately 140 cycles, and its effect is the wavy opening characteristic shown in curve Ain Fig. 3.'-

Curve A also shows the slow starting action of the shutter at the beginning of a signal, which is considerably shortened by the present invention without yintroizlucing thumps, as will be explained hereinafter. f

To neutralize or overcome the load resonance, a series tuned filter tuned to the resonance frequency of the load is shunted across the shutter winding I5 and includes an inductance |1, a condenser I8, and a resistor I9. Where the load resonance occurred-at 140 cycles, the inductance I1 was approximately one henry and the capacity approximately 1.5 mf. To match the Q factor of the series tuned circuit to the Q'of the shutterv load, the resistor I9 had a value of approximately- This shunt circuit was found to oom- 75 ohms. pletely damp the shutter so that no oscillation was introduced. as shown by curve B in Fig.l 3. To increase the starting speed of the opening of the shutter or galvanometer, as shown by the toe of curve B, a condenser 2| isk connected in shunt to resistor I2 in the timing circuit proper, this capacitor functioning to permitfthe charging voltage to be impressed upon the condenser 9 simultaneously with itsimpression on condenser 8, and consequently, on the grid of the tube I4. In this manner, the starting delay action of the resistor I2 at the beginning of the signal is removed, the condenser 2| being given afvalue 4 intensity or length of the recording light beam 1s controlled by the shutters 3|, while the light is varied in accordance with the instantaneous values of the signal variations by the galvanometer 21.

Bridged between the amplifiers 24 and 25, is a noisereduction amplifier which includes an input transformer 40 across the secondary of which is a unitary `double potentiometer having resistances 4|-42 of 50,000 ohms each, to which is connected a push-pull amplifier having vacuum tubes 44 and 45. The output of the push-pull amplifier is connected through the transformer 41 to a full-Wave rectifier tube 48, which may be which will not affect the overall timing of the circuit or interfere with its ability to reduce the amplitude of the rectifier ripple components. In

a typical circuit where condenser 8 had a value of .03 mf., condenser 9, a value of .003 mf.,l resistor I2, a value of 4 megohms, and resistor I I, a value of 1.5 megohms, the condenser 2| had a value of .001 mf.y Although the action of the series tuned circuit |1|8|9 has a slight tendency to slow the opening time, the useof condenser12| notonly overcame this tendency, but reduced the starting time from 5 milliseconds to 1.5 milliseconds to provide a faster openingandthusminimize peak clipping.

Referring now to Fig. 2, a noise reduction circuit is shown in which the features shown in Fig.

1 are included in addition to a rectifier-filtercombination and a linear direct current amplifier to-provide an improved marginfor the/noise reduction action. Diagrammatically shown inFig. 2, is a microphone 23 connected to a preamplifier 24, the output of which is impressed on an amplifier 25 connected to the modulating coil of agalvanometer 21...v The galvanometer is showntinv a sound recording system diagrammatically rep-fresented by a light source 29, a mask opticalV unit 30, a, noise reductionshutter unit 3|, a slit optical unit 33, and a film 34.

This recording system is atypical commercial type wherein the average.

lof the RCA GHG type.

Connected across the secondary of the transformer 41 and the center tap which feeds the rectifier tube 48, are three selenium rectiers 50, 5|, and 52 with a load resistor 53, the purpose of which will be explained hereinafter. The output of the rectifier is connected to a linear direct current amplifier tube of the RCA 6V6 type, through a timing circuit comprising shunt condensers 51 and 58, series condenser 59, and resistors 6| and 62. 'I'he output of the direct current amplifier is impressed on the actuating coil of the shutters 3| over a circuit from the cathode of tube 55 and conductors 64 to the anode potential supply and the anode of tube 55. A variable resistor of approximately 1000 ohms is used in the cathode circuit to adjust the initial shutter opening to the proper amount. A voltage regulator tube 61 is provided, which, in the present circuit, reduces a potential of 250 volts at the B plus terminal to 145 volts for the screen grid of tube 55 and the plates and screens of tubes 44 and 45.

It will be noted that two of the features of Fig. 1 are provided in the circuit of Fig. 2; namely, a series tuned circuit shunting the winding of shutters 3| and including an inductance 69, a resistor 10, and a capacitor 1|, these units having the same values as in Fig. 1 when the resonance peak of the shutter load occurs at approximately cycles. This shunt circuit damps and stabilizes the shutter action during increases in the signal currents to provide the straight portion of curve B in Fig. 3. Also in the timing circuit of' Fig. 2, is the shunt condenser 59, which speeds up the initial shutter action as shown in thetoe portion of curve B. Typical values for this timing'flter are: condenser 51, .01 mf.; condenser 58, .003 mf.; condenser 59, .001 mf.; resistor 6|, 2 megohms; and resistor B2, 20 megohms.

Referring now to the rectifier circuit, it will be noted that the rectifier not only provides for full-wave rectification, but also that the control grid of the direct current amplifier is connected to the anodes or plates of the rectifier without going through the secondary winding of the transformer. This connection provides a negative potential on the control grid of the tube 55 in the usual manner, but avoids the effect of leakage in the windings of the transformer 41. In prior noise reduction amplifiers Where the anode or anodes of the rectifier were connected to the timing circuit through the winding of the transformer, the direct current resistance between the winding and other components of the transformer provided a leakage path which varied,- `with different transformers and with humidity v directly to the timing circuit and because of the excellentnsulationbetween the anodesiandfother components' of; thea-rectier; the; dischargefresist ance; is maintained; at a constant value.- It;` will:

fbeinotedthat theymidtapzo the transformer 41 :is

substantiallyiat-ground potential' and differs: therefrom only'fbyftheimpedance of the' shutter-1 windingvwhich is approximatelyy ohms.

too long afclosing time: andl permits a .swishing noise to be introducedat-the end ofY asignal.-A

The Haynes patent discloses several types .ofI circuits ;for-.decreasing this closing 1 time byf making. thedischarge -characteristiczlinear instead of: ex-` ponential. He accomp1ishes-this by,` the use of'v an additional shunt rectiiier;witha,fixednpotenfl tial-fbias,- The-present circuit accomplishes the samevclosingtime-in-a similar manner, but does.- -sowlth the same rectifier used rfortherectical tion of the signal currents. InqFig, 4, thenormall dischargeftime of the condenser is shownbyvvcurve C;v However.,v only` the-straight portion of j thef curve.A shown above the bias lineyD Ais desired lto'v permit the shutter to close linearly. That i1s,with the maximum negative;` charge,- the; shutter is wide. open, and-at-zero; charge, it is closed,the,- curved-portion of curve-C being v,eliminated bythe f use ofthe Abias `and the Voltage-'limiter-action of' therectiiier 48.

.In the; pre-sentfcircuit, thisibiasis obtained over resistor 62. and conductor I3 connected ltothev 145 voltf side of lthe voltage` regulator, tube Y61. Thus,v at the-cessation. of aysignal, theyshutter 'move-e ment will be linear to the zero charge-,point of thefcondenser-Bl after. which the rectier 48 will transmit current from the B supply and prevent the charge onxcondensers:l'iflandiil from becoming., positive. TheY highl valueV of resistorA 62 lengthens the linear portion .ofjthe discharge..

curve to correspond to the desired length of closing time; namel'y,- about-200f'milliseconds The-shutter action in many; noise reduction, systems athigh signal levelsisasymptotic Vtofull open position, as shown by thepupper portionof curve A and the dotted lines from curve B in Fig. 3. This is due to the non-linearity of the rectiiier and the exponential characteristic of the direct current amplifier. To obtain :a better margin at high amplitudes, a reversal of the curvature of the characteristic between the input to the rectifier and the output of the direct current amplifier is desired. In the present circuit, this is accomplished by the use of selenium rectifiers 50, 5I, and 52 as non-linear resistances, and the current limiting resistor 53 connected as shown in Fig. 2. The polarization of rectiiiers 50, 5I, and 52 is such as to neutralize or compensate for not only the non-linearity of the rectifier 48 but actually over-compensates to produce a larger margin at low level signal inputs than at the higher levels. This reduces the-amount of clipping at low levels where clipping is most noticeable.. Although a similar action could loe obtained by the use of two reversed rectifiers shunted across the full secondary winding of the transformer, the rectiiiers thus having the full voltage of the secondary impressed thereon, the present bridge arrangement reduces the voltage on each rectifier to one-fourth of the total voltage. Thus, rectiers and 5I need not be as accurately balanced or matched while rectifier 52 may have any value, since it is common t0 looth rectiflers.

To maintain this relationship, an RCA 6V6 vacuum tube is used as a direct current amplier, this tube providing substantially linear ampiificationas comparedwithiprevicusly used-tubes; RCA?. 6Kls., overall" relationship f betweenf input; at@ 4 andoutput; at; 64 provides the f upper.: solid: line; lfioulderl to thev curve B4 in Fig,` 3 -irr-p` Thewcircuit above-described eliminates theA rip-'- ple causedi by "resonance of the load; reducesitheA startintime.; of the` noise reduction actionto;A reduce peak g clipping t0- a minimum, and `has. Y a;

characteristic tor-provide a` desirable margini Inziv oneiadjustmentof the circuit; it waslfoundathat; the overall noise reduction characteristic.; asu shownnbygcurve ABinllig. 3, approximatesqapure 50cyclesine-wave; that is, the lower` toefhasarv 1 curvature corresponding itc; that: oi',l a" 50 Cycle? wave;l thev linearsection of the curveghas a slopej corresponding-to a 50.,l cycle-wave, and the upper; shoulder. odiv the curve has ',a-curvature correspond#- ing ytciless than a 50 cycle wave.- Thisresultsgin; a. noise reduction `envelope which Acan containpnoy; frequenciesiaibove 50wcyc1es;` and sincemany c01I1:.- mercial `types; of soundf reproducing, systems,` cutz'. offfat iilecyclesvorraloove;` no sound `caused` by the noise-reduction action1willbe-reproduced ,by suchf systems; This; frequency: Camof course; be'. adfi ju-sted-to suit any'other lower cutoif frequency-f of :the reprod doing-system;v

Certain'ffeatures of the inventionl may began-1 pliedtofpresent types of noisefreductionsystems,r` as, shown inA Fig.` 1;. or; all fea-tures maybe ine'or-fv poratedf in the; noise reduction;` circuit lshown; in-l Fig; 2'. ,I

lclaimr as;my= invention:

1'.; AAvy noise reductionfamplier system;compris;-.4

ing as rectifier, a timing circuit connected to-tire out-putfofisaid: ,ife,-cti1"ier1,r a direct current. amplifier.l connected to said timing:circuit,ia loadcircuit for; utilizing the output of saidfampliiier,,saidiload'Y circuitihavingfa;V resonant frequency, and means connectedfacross saidload circuit'.forxeliminating; resonant'y frequency: currentsfromN said load: ciraj cuit;l

2'. Annoise reduction amplier system inv aca. cordance with claim 1,. in which said timingf-cin-t` cuit includesa `pluralityof shuntingk condensers separatedebvaresistorand a1 condenser in shunt to'v .said resistor to: fpermitthe; initial charging' off. said shunting condensers to;:besimu1taneous.'`

3. A noise reduction circuit comprising a pushpull alternating current amplifier, a full-wave rectifier connected to the output of said amplifier, a direct current amplifier, a capacitance-resistance timing circuit for directly interconnecting the anodes of said rectifier with the control grid of said direct current amplifier, a load circuit connected to the output of said direct current amplifier, and a tuned circuit in shunt to said load circuit for eliminating from said load circuit currents of a frequency to which said load circuit is resonant. l

4. A noise reduction circuit comprising a push pull alternating current amplifier, a full-wave rectifier connected to the output of said amplifier, a direct current amplifier, a capacitance-resistance timing circuit for directly interconnecting the anodes of said rectifier with the control grid of said direct current amplifier, and a plurality of rectiers bridged between said alternating current amplifier and said full-wave rectier, said plurality of rectiiiers being polarized to over-compensate for the non-linearity of said full-wave rectifier to produce a predetermined overall relationship between the input to said pull alternating current amplifier, a full-wave rectifier connected to the output of said amplifier, a direct current amplifier, a capacitance-resistance timing circuit for directly interconnecting the anodes of said rectifier with the control grid of said direct-current amplifier, and a bias source of potential for said interconnecting timing circuitto utilize only the straight portion of the discharge characteristic of one of said condensers, said full-wave rectifier passing current from said bias source to prevent a reversal of polarity on said condenser.

6.V A noise reduction circuit comprising a fullwave rectifier, a direct current amplifier, a resistive-capacitive filter interconnecting the output of said rectifier and the input of said amplifier, the anodes of said rectifier being connected directly to said filter, a load circuit connected to the output of said amplifier, said filter including a shunt condenser, a shunt resistor, a series resistor shunted by a. condenser, and a source of bias potential in series with said shuntresistor.

7. A noise reduction circuit comprising a fullwave rectifier, a direct current amplifier, a resistive-capacitive filter interconnecting the output of said rectifier and the input of said amplifier, the anodes of said rectifier being connected directly to said filter, a load circuit connected to the output of said amplifier, an input transformer for said full-wave rectifier, the-cathodes of said rectifier being connected across the full secondary winding of said transformer, and three additional rectifiers, two of said additional rectifiers being connected in series and in opposing polarity across said secondary and the third of said additional rectifiers being connected between the mid-tap of said secondary and a point between said two rectiiiers.

8. A noise reduction timing circuit comprising a pair of shunting condensers, a series resistor between said condensers, a condenser in Ashunt to said resistor for initially charging said shunting condensers simultaneously, a shunting resistor, a source of potential for said shunting resistor, and means in series with said shunting resistor for preventing the charge on said shunting condenser from reversing in polarity when the charge on said condensers reaches Zero.

' 9. A noise reduction circuit for a sound re cording system, said circuit comprising an alternating current amplifier, a rectifier connected tov said amplifier, a timing filter connected to said rectifier, a direct current amplifier connected to said filter, a noise reduction-element connected to said direct current amplifier, the input to said filter being connected directly to the anode of said rectifier, and a plurality of additional rectifiers bridged across said first Vmentioned rectier for over-compensating for the non-linearity of said first mentioned rectifier.

10. A noise reduction circuit for a sound recording system, saidrcircuit comprising an alternating current amplifier, a rectifier connected to said amplifier, a timing filter connected to said rectifier, a direct current amplifier connected to said filter, and a noise reduction element connected to said direct current amplifier, the input to said filter being connected directly to the anode of said rectifier, said timing filter having a series resistor, a plurality of shunting condensers, and a condenser shunting said resistor to permit said shunting condensers to be initially charged simultaneously.

11. A noise reduction circuit comprising a fullwave rectifier, a resistance-condenser filter connected to the output of said rectifier,- said filter having a plurality of shunting condensers and a resistor, a series resistor shunted by a condenser, a direct current amplifier connected to said filter, a noise reduction shutter unit connected to said amplifier, and a series tuned circuit shunting said shutter unit and tuned to the resonant frequency of said shutter.

ALEXIS BADMAIEFF.

REFERENCES ,CITED UNITED STATES PATENTS Number Name Date Y 2,357,661 Kellogg Sept. 5, 1944 2,359,989

Haynes Oct. 10, 1944 

